Understanding IPv6-PD over PPPoE




I'm trying to get to grips with IPv6-PD over PPPoE.

I'm on CentOS 8 (NetworkManager 1.30) and have main.dhcp=dhclient. My PPPoE connection is set to ipv6.method=auto.

With no NICs set to ipv6.manual=shared, I can see from a traffic dump that:

- PPP link comes up and assigns IPv4 through LCP.
- I send an IPv6 router solicitation to my ISP.
- ISP responds with a router advertisement, which has the "managed address configuration" and "other configuration" flags set.
- I send DHCPv6 Solicit with an IA_NA option.
- ISP responds with a DHCPv6 Advertise, which does NOT contain an IPv6 address.

I end up with an IPv6 default route pointing at the PPP interface, but no public IPv6. IPv6 traffic from me to a public address ends up with my link-local address as the source, which is obviously broken.


Conversely, with one or more NICs set to ipv6.method=shared (even if its only a dummy NIC), I get:

- PPP link comes up and assigns IPv4 through LCP.
- I send an IPv6 router solicitation to my ISP.
- ISP responds with a router advertisement, which has the "managed address configuration" and "other configuration" flags set.
- I send DHCPv6 Solicit with IA_NA and IA_PD options.
- ISP responds with a DHCPv6 Advertise, which does NOT contain an IPv6 address, but does contain my /48 public prefix.

Again, the PPP NIC ends up with no public address, but now the "shared" NIC gets a public v6 address. IPv6 traffic from me to a public address ends up with the "ipv6.method=shared" NIC's address as the source, as this is the only public v6 address available.


Is my ISP behaving in a broken way, by not assigning me an address in response to IA_NA, or is it standard behaviour for ISPs to only respond to IA_PD requests?

It feels as though NetworkManager should always be making both IA_NA and IA_PD requests and, if it didn't receive an IA_NA response from the ISP, it should assign an address from the delegated prefix to the PPP interface. This would prevent the situation where a machine with no "ipv6.method=shared" NICs ends up with no public v6 address at all.


Any insight would be appreciated - many thanks. :)

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- Steve Hill
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